1. Field of the Invention
The present invention relates to a linear actuator, and particularly a linear actuator to convert a rotational movement into a linear movement.
2. Description of the Related Art
A linear actuator is conventionally provided which, for example, converts a rotational movement of a motor into a linear movement of an output shaft.
For example, in a linear actuator disclosed in Japanese Patent Application Laid-Open No. 2002-122203, the outer periphery of an output shaft is provided with a screw thread while the inner periphery of a nut as a mating member fixedly attached to a rotor of a motor is provided with a screw thread, wherein both of the screw threads engage with each other whereby the rotational movement of the rotor is converted into the linear movement of the output shaft in the axial direction.
Also, in a linear actuator disclosed in Japanese Patent Application Laid-Open No. 2002-372117, a ball screw system is provided between a rotor and an output shaft, whereby the rotational movement of the rotor is converted into the linear movement in the axial direction.
In the linear actuator disclosed in Japanese Patent Application Laid-Open No. 2002-122203, the rotational movement of the rotor is transmitted to the output shaft by means of a screw system which has a high friction resistance, and therefore the transmission efficiency is low thus preventing the torque of the motor from being transmitted sufficiently.
On the other hand, in the linear actuator which is disclosed in Japanese Patent Application Laid-Open No. 2002-372117, and which uses a ball screw system, the output shaft is provided with a ball screw, and also a place corresponding to a nut is provided with a ball groove, wherein balls must be circulated without running off from the screw portion, thus resulting in a complicated structure.